A general capacitor discharge ignition device for an internal combustion engine is comprised of an exciter coil that is provided in a magneto generator driven by an internal combustion engine and induces an AC voltage in synchronization with rotation of the engine, an ignition capacitor that is provided on a primary side of an ignition coil and charged with one polarity with a positive half cycle of an output voltage of the exciter coil, a thyristor that is turned on when a trigger signal is provided to discharge charges in the ignition capacitor through a primary coil of the ignition coil, and a thyristor trigger circuit that provides the trigger signal to the thyristor at an ignition position in the internal combustion engine.
As disclosed in Japanese Patent Laid-Open Publication No. 59-41669, a magneto generator that comprises a magnet field on an outer periphery of a flywheel has been often used as a magneto generator having an exciter coil as described above. Such a magneto generator includes a magneto rotor that forms a magnet field with three poles by attaching a permanent magnet to an outer periphery of a flywheel, and a stator comprised by winding an exciter coil around a core having magnetic pole portions facing the magnetic poles of the magnet field of the magnet rotor, and generates one-and-a-half cycle of an AC voltage constituted by a positive half cycle of an output voltage and first and second negative half cycles of output voltages generated before and after the positive half cycle of the output voltage, respectively, at least once from the exciter coil during one rotation of a crankshaft.
In a capacitor discharge ignition device for an internal combustion engine using such a magneto generator, as disclosed in Japanese Patent Laid-Open Publication No. 59-41669, an ignition capacitor is generally charged with a positive half cycle of an output voltage generated by an exciter coil to provide a trigger signal to a thyristor by using a negative half cycle of an output voltage generated by the exciter coil.
If the capacitor discharge ignition device for an internal combustion engine using the above described magneto generator is comprised so that the trigger signal is provided to the thyristor by the negative half cycle of the output voltage generated by the exciter coil, charges in the ignition capacitor are discharged through the thyristor for an ignition operation when the trigger signal is provided to the thyristor by an output of a second negative half cycle of an output voltage. The trigger signal is also provided to the thyristor when a first negative half cycle of an output voltage is generated before the exciter coil generates the positive half cycle of the output voltage, but at this time, the charges have not yet been stored in the ignition capacitor, and thus the thyristor does not conduct to cause no ignition operation.
In the capacitor discharge ignition device for an internal combustion engine using the magneto generator having the magnet field with the three poles on the outer periphery of the rotor, when the trigger signal is provided to the thyristor by using the negative half cycle of the output voltage of the exciter coil, the trigger signal is also provided to the thyristor when the first negative half cycle of the output voltage is generated before the exciter coil generates the positive half cycle of the output voltage, but the trigger signal has to be eliminated before the positive half cycle of the output voltage of the exciter coil rises. If the trigger signal is provided to the thyristor when the positive half cycle of the output voltage of the exciter coil rises, the thyristor conducts to short-circuit the exciter coil, thus preventing charging of the ignition capacitor to cause misfire of the engine.
When a sufficient space can be provided between magnetic poles of the magnetic rotor, and a crank angle position where the exciter coil generates the first negative half cycle of the output voltage can be sufficiently separated from a crank angle position where the exciter coil generates the positive half cycle of the output voltage, no trigger signal is provided to the thyristor when the positive half cycle of the output voltage of the exciter coil rises.
However, when the space between the magnetic poles of the magnet rotor has to be narrowed, and the crank angle position where the exciter coil generates the first negative half cycle of the output voltage is close to the crank angle position where the exciter coil generates the positive half cycle of the output voltage, such as when the rotor of the magneto generator has to have a smaller outer diameter, a time is reduced from when the exciter coil generates the first negative half cycle of the output voltage to when the exciter coil generates the positive half cycle of the output voltage during high speed rotation of the engine, and thus a trigger signal current provided to the thyristor by the negative half cycle of the output voltage generated before the positive half cycle of the output voltage may remain when the positive half cycle of the output voltage rises.
Such a state causes the thyristor to conduct to short-circuit the exciter coil when the exciter coil generates the positive half cycle of the output voltage, thus preventing charging of the ignition capacitor to cause misfire of the engine.
Thus, in the capacitor discharge ignition device for an internal combustion engine in which the magneto generator having the exciter coil that generates one-and-a-half cycle of the AC voltage constituted by the positive half cycle of the output voltage and the first and second negative half cycles of the output voltages generated before and after the positive half cycle of the output voltage, respectively, at least once during one rotation of the crankshaft of the internal combustion engine is used to provide the trigger signal to the thyristor by the negative half cycle of the output voltage of the exciter coil, if the space between the magnetic poles of the magnet rotor is narrowed, and the crank angle position where the exciter coil generates the first negative half cycle of the output voltage is brought close to the crank angle position where the exciter coil generates the positive half cycle of the output voltage, the rotational speed of the engine is limited since the charging of the ignition capacitor is prevented during the high speed rotation of the engine.